Conventionally, in electronic computers including personal computers, a data storage circuit composed of a combination of a large number of storage elements is provided in a semiconductor device such as a CPU and a memory IC. Data are stored in the data storage circuit so as to execute various types of processing.
The storage elements in such a data storage circuit are normally configured so that 1-bit data is stored in each of the storage elements. More specifically, the storage element is capable of keeping two different states. When one state is represented by “0” and the other state by “1”, data of “0” or “1” is stored by keeping any one of the states. A large number of such storage elements are provided to enable the storage of data in an amount corresponding to the number of provided storage elements.
Storage elements having a wide variety of structures are known as such storage elements. For example, a storage element of a flash memory, which consists of a N-channel MOSFET (Metal Oxide Silicon Field Effect Transistor), is capable of storing data “0” and “1” while a state where charges are accumulated in a floating gate layer provided for a gate electrode section is represented by “1” and a state where no charge is accumulated in the floating gate layer is represented by “0” or vice versa.
A storage element of a MRAM (Magnetic Random Access Memory), which consists of a ferromagnetic tunnel junction element, is capable of storing data “0” and “1” while a state where the direction of magnetization of a free magnetization layer is anti-parallel to that of a fixed magnetization layer is represented by “1” and a parallel state by “0” or vice versa.
In a data storage circuit provided with these storage elements, in a case where data of “0” or “1” is newly stored in a certain storage element, a state of change is induced by application of a predetermined voltage to the storage element or by allowing a current to pass therethrough. At this time, the storage element is forced into a predetermined state by application of a predetermined voltage to the storage element or by allowing a current to pass therethrough, regardless of whether existing data that is previously stored in the storage element is “0” or “1”, thereby storing new data.
In a data storage device consisting of a semiconductor device including the conventional data storage circuit described above, however, new data is written by application of a voltage to the storage element or by allowing a current to pass therethrough even though the previously stored existing data and the new data to be written are identical. Therefore, there is a problem that substantially ineffective power consumption is generated which hinders power saving.
In particular, the storage element used for the flash memory or the MRAM described above requires a considerable amount of electric power in view of its structure to cause a change of state in order to write new data. Therefore, wasteful power consumption is increased in the data storage circuit, such as the flash memory or the MRAM, by the corresponding amount to further hinder power saving.